1. Field of the Invention
This invention relates to an electrophotographic apparatus, and more particularly to improvements in an electrophotographic apparatus wherein an electrostatic latent image formed on a latent image bearing member is developed by liquid developer and then any unnecessary developing liquid is removed from the latent image bearing member.
2. Description of the Prior Art
Generally, in the wet development type electrophotographic apparatuses, it has been practised to remove any excess developing liquid remaining on the surface of the latent image bearing member after the development. This is for the following reason. In the apparatuses of the type wherein the developed toner image is transferred to paper or other transfer medium, when the transfer medium is urged against the latent image bearing member having a great deal of developing liquid thereon, there may occur a violent flow of liquid which may disturb the toner image electrostatically adsorbed to the bearing member, or toner particles suspended in a thick layer of liquid may stick to the transfer medium to cause fog, or a great deal of liquid may stick to the transfer medium to cause the need for a great deal of heat to dry the transfer medium, which in turn may result in production of a lot of air-contaminating gases. Also, in the apparatuses of the type wherein the developed toner image is directly fixed on the latent image bearing member, if the fixing process is effected with a great deal of developing liquid remaining on the latent image bearing member, there may likewise occur fog and a great deal of heat may be required for drying which may also result in undesirable production of much noxious vapor.
There is a diversity of electrophotographic apparatuses in which unnecessary developing liquid is removed after the development, and examples of the electrophotographic apparatus of the type similar to the present invention in which the means for forming the power of squeezing the unnecessary developing liquid is not in contact with the developing liquid are disclosed in U.S. Pat. Nos. 3,369,918, 3,627,410 and 3,741,643, or U.S. Pat. Nos. 3,722,994 and 3,760,152, etc. The first three patents pertain to the apparatus in which compressed air is blown against the developing liquid on the latent image bearing member to remove the unnecessary developing liquid, and the latter two patents pertain to the apparatus in which corona discharge is applied to the developing liquid to squeeze and remove the unnecessary developing liquid. In any of these patents, the means for producing the squeeze power (such as corona discharge electrode or air injection nozzle) is spaced apart from the developing liquid and thus, from the latent image bearing member, and this avoids the inconvenience that such means mechanically injures the latent image bearing member or the toner image formed thereon, whereas thereis left the disadvantage that the squeeze power tends to become irregular with respect to the widthwise direction of the latent image bearing member, namely, the direction perpendicular to the direction of movement of the latent image bearing member relative to the squeeze power forming means. This is attributable to the extreme difficulties encountered by the air knife developing liquid removal device in forming an air stream injected under uniform pressure with respect to the widthwise direction of the latent image bearing member, or to the difficulties encountered by the corona discharge developing liquid removal device in producing uniform corona discharge with respect to the widthwise direction of the latent image bearing member.
If the squeeze power becomes irregular with respect to the widthwise direction of the latent image bearing member as described, the thickness of the developing liquid layer squeezed and flowing down the latent image bearing member may also become irregular, so that streak-like density irregularities may occur to the toner image formed on the surface of the latent image bearing member. The reason for this is that toner particles still available for the development are suspended in the developing liquid layer squeezed and flowing down the latent image bearing member and the quantity of such toner varies in accordance with the variation in thickness of the liquid layer. Also, the thickness of the squeezed liquid becomes irregular and accordingly, the thickness of the developing liquid layer left on the latent image bearing member necessarily becomes irregular with respect to the widthwise direction of the latent image bearing member and this causes variations in the electrical drifting force of the toner in the liquid during the transfer of the toner image onto paper or the like, which in turn may adversely effect the quality of the transferred image.
Therefore, in the electrophotographic apparatus using corona discharge to remove the unnecessary developing liquid, it has heretofore been the practice that an insulative converging plate disposed adjacent to the opening of the shield member of the corona discharger to permit a corona discharge flow to be applied to a region of narrow width at a high density (see U.S. Pat. No. 3,760,152) is brought into uniform contact with the developing liquid dammed up and bulged by the application of the corona discharge, thereby the thickness of the squeezed developing liquid layer is uniform or even. According to this, however, a long time has been required from the initial contact of a portion of the squeezed developing liquid with the converging plate till the completion of uniform contact of such liquid with the entire surface of the converging plate, and much time has also been necessary from the stoppage of the corona discharge till the drain-off of the developing liquid from the converging plate. That is, in addition to the time required for the intended image processing steps such as development and transfer, it has been necessary to provide a long preparation time for uniforming the thickness of the squeezed developing liquid and a post-process time for draining the developing liquid off the surface of the latent image bearing member (if this latter time is long, some of the developing liquid will inconveniently dry up and stick to the surface of the latent image bearing member). Moreover, there has been such a disadvantage that the developing liquid comes into the interior of the corona discharger from the edge of the opening of the converging plate to contaminate the shield member and/or the discharge electrode to reduce the squeeze power.